1. Field of the Invention
This invention relates to membrane structures for separation of materials on a molecular scale, such as selective sorption, molecular sieving, and the like, and more particularly to modification of such membranes by atomic layer controlled chemical vapor deposition to increase effectiveness of the membrane for materials separation, or other purposes.
2. Description of the Prior Art
Materials, such as gases or liquids, can be separated from other different materials on the molecular level by a number of techniques. Some well-known examples include evaporation and condensation, which is used to distill alcohol, electrolysis, which is used to remove chromium from solution and plate it onto parts, and crystallization, which is used to purify salts, drugs, or other substances from contaminated mixtures or solutions. More relevant to this invention, thin membranes of some solid materials that have very small pores are permeable to certain gases or liquids comprising small molecules, and impermeable to other gases or liquids that may comprise larger molecules, different shaped molecules, molecules with different polarizability, different absorption properties, or combinations of any of these factors. In industrial applications, organic polymer membranes are used, for example, to separate substances such as ethanol from water. Also of interest, some solid materials are adsorbents for certain gases or liquids. Some zeolites, such as 4A, are more adsorbent of molecular nitrogen (N.sub.2) than of molecular oxygen (O.sub.2) at increasing pressures and desorb the nitrogen when pressure is decreased. This adsorption/desorption cycle is used in combination with reversing flow directions to separate nitrogen gas (N.sub.2) from oxygen (O.sub.2) in air.
Recognizing how such molecular separation materials and mechanisms work, it is, of course, desirable to improve them to make them more effective and more efficient for their material separation functions. For example, in the U.S. Pat. No. 3,837,500, issued to Nichols et al., a process is described for reticulating or cross-linking an organic porous polymer membrane comprising ultrathin layers of polyvinyl alcohol and polyvinyl pyrrolidone with diisocynate reticulation agent to decrease and control the pore size of the membrane for more effective water/salt separation.
Zeolites are microporous crystalline alumina silicates with a narrow distribution of pore sizes on a molecular scale, and they have high thermal, chemical, and mechanical stabilities. Molecular sieves can be, for example, alumina phosphates (ALPO) or silicaaluminaphosphates (SAPO), which are also microporous, crystalline materials with a narrow distribution of pore sizes and also have high thermal, chemical, and mechanical stabilities. Therefore, zeolites and molecular sieves can be used not only for gas separations in adsorption/desorption processes, as mentioned above, but also as diffusion membranes when prepared in thin film form. The size and adsorption properties of the zeolite pores, however, limit what can be separated with a particular type of zeolite membrane, even if the crystalline structure is perfect and defect free. However, perfect and defect free zeolite crystalline structures are not readily available or easy to prepare, so most zeolite materials have defects and separations or spaces between crystals, which can be larger than the pore sizes in the crystalline structures. Therefore, transport of molecules by diffusion can take place both within the zeolite crystals and between adjacent crystal faces. Since the spaces between adjacent crystal faces can be larger than the pores in the zeolite crystals themselves, it is very difficult to produce zeolite membrane types with good separation capabilities, and only a few have been prepared prior to this invention.
There is a substantial need for better and different zeolite membranes. For example, the sequential adsorption/desorption pressure swing cycles mentioned above for separating gases require a high capital investment for any significant volume production in industrial plants, and such systems have high maintenance costs. A gas separation system that takes advantage of both the adsorption and diffusion properties of zeolites, but operates at steady state rather than pressure cycling would be much more efficient, but such a system would require good, effective, and efficient continuous zeolite membranes.
In recent years, thin, dense layers of zeolites have been prepared both in self-supporting thin film membrane structures and on macroporous substrates or supports. See, for example: W. J. W. Bakker et al., "Single and Multi-Component Transport Through Metal Supported MFI Zeolite Membranes," Precision Process Technology, Eds. M. P. C. Weiznen and A. A. H. Drinkenburg, Lluwer Academic Publishers, 1993, page 425; W. J. F. Bakker et al., "Doorbreak in Ontwikkeling Zeolietmembranen, in Dutch (English translation of title: Break-Through in Development of Zeolite Membranes," Proces Technologies, Vol. 3, December 1993, page 7; M. Jia et al., "Ceramic-Zeolite Composite Membranes and Their Application for Separation of Vapor/Gas Mixtures," J. Membr. Sci., Vol. 90, 1994, page 1; Y. Yan et al., "Zeolite ZSM-5 membranes grown on porous .alpha.-Al.sub.2 O.sub.3," J. Chem Soc. Chem. Commun., 1995, page 227; T. Sano et al., "Potentials of Silicalite Membranes for Separation of Alcohol/Water Mixtures," Studies in Surface Science and Catalysis, 1994, vol. 84, page 1175; J. Tsikoyiannis et al., "Synthesis and characterization of a pure zeolite membrane," Zeolites, February 1992, vol. 12, page 126; E. Wu et al., "Hydrocarbon adsorption characterization of some high silica zeolites," Studies in Surface Science and Catalysis, 1986, vol. 28, page 547; and S. Xiang et al., "Formation and characterization of zeolite membranes from sols," 3rd International Conference on Inorganic Membranes, Worcester, Mass., Jul. 10-14, 1994. Hydrothermal synthesis with aqueous solutions of zeolite precursors is the most widely used method to form the membrane layer. See, for example: M. Jia et al., supra,; T. Sano, supra; E. Geus et al., "High temperature stainless steel supported zeolite (MFI) membranes: preparation, module construction, and permeation experiments," Microporous Materials, 1993, vol. 1, page 131; Y. Yan et al., "Preparation of zeolite ZSM-5 membranes by in-situ crystallization of porous .alpha.-Al.sub.2 O.sub.3, Ind. Eng. Chem. Res., 1995, vol. 34, pages 1652-1661; Y. Yan et al., "Zeolite ZSM-5 Membranes Grown on Porous .alpha.-Al.sub.2 O.sub.3," J. Chem. Soc., Chem. Commun., 1995, pages 227-228. Other techniques for forming zeolite membranes may include: (i) treating a dry layer of zeolite precursors in steam, N. Nishiyama et al., "A defect-free modernite membrane synthesized by vapor phase transport method," J. Chem Soc. Chem. Commun., 1995, page 1967; (ii) embedding zeolite crystal in a metal matrix, P. Kolsch et al., "Zeolite-in-metal membranes: Preparation and testing," J. Chem. Soc. Chem. Commun., 1994, vol. 21, page 2491; and (iii) sintering zeolite crystals to form a dense layer, C. Engelen et al., "Membrane for separation of small molecules and its manufacture," PCT Int. Appl. WO 93/19841, 1993, to ECN. Such zeolite membranes prepared by these processes have shown a good potential for the separation of molecules that have sufficient differences in their respective adsorption and diffusion behavior. Other commercially important separations, such as nitrogen from carbon dioxide (N.sub.2 /CO.sub.2) or nitrogen from methane (N.sub.2 /CH.sub.4) have not been obtained with these zeolite membranes, most likely because the CO.sub.2 and CH.sub.4 molecules are small and can easily enter and permeate through the regions between the zeolite crystals. It is noteworthy that polycrystalline silicalite membranes have been modified with silane coupling reagents to improve pervaporation performance with aqueous ethanol solutions. The modification was performed from the liquid phase with a large alkyltrichlorosilane molecule (alkyl=C.sub.8 H.sub.17 and C.sub.18 H.sub.37). The coupling agents were dissolved in an inert solvent. The authors concluded that the change in permeation properties was caused by an altered hydrophobicity of the external membrane surface. See T. Sano et al., "Improvement of the pervaporation performance of silicalite membranes by modification with a silane coupling reagent," Microporous Materials, 1995, vol. 5, pages 179-184. However, the coupling agents used were longer molecules that were not able to enter the zeolite pores and may be too large to enter gaps between crystals. Also, diffusion and transport in the fluid phase is different than in the gas or vapor phase, so different membrane regimes should be accessed at different rates.